Energy 57 (2013) 76e84 Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy The feasibility of synthetic fuels in renewable energy systems Iva Ridjan a,*, Brian Vad Mathiesen a, David Connolly a, Neven Duic b a Department of Development and Planning, Aalborg University, Denmark b Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Zagreb, Croatia article info abstract Article history: While all other sectors had significant renewable energy penetrations, transport is still heavily depen- Received 9 October 2012 dent on oil displaying rapid growth in the last decades. There is no easy renewable solution to meet Received in revised form transport sector demand due to the wide variety of modes and needs in the sector. Nowadays, biofuels 23 January 2013 along with electricity are proposed as one of the main options for replacing fossil fuels in the transport Accepted 24 January 2013 sector. The main reasons for avoiding the direct usage of biomass, i.e. producing biomass derived fuels, Available online 26 February 2013 are land use shortages, limited biomass availability, interference with food supplies, and other impacts on the environment and biosphere. Hence, it is essential to make a detailed analysis of this sector in Keywords: Synthetic fuels order to match the demand and to meet the criteria of a 100% renewable energy system in 2050. The fi 100% renewable energy system purpose of this article is to identify potential pathways for producing synthetic fuels, with a speci c focus Transport sector on solid oxide electrolyser cells (SOEC) combined with the recycling of CO2. Feasibility study Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Future energy systems will be based on high shares of fluctu- ating renewable energy sources and the conversion of electricity Shifting from oil to other fuels is not just desirable, it is neces- into various energy carriers will become the main concern. sary for a number of reasons: resources are limited, geographic Research shows that 100% renewable energy scenarios are techni- distributions are uneven and the greenhouse gas emissions must be cally possible in the future [4,5]. The change from conventional reduced. The transport sector is one of the most important sectors energy systems to renewable energy systems reduces greenhouse of our time, as well as a significant carrier and the backbone of the gas emissions, has a positive socio-economic effect, and can create economic and social development in many countries. With a new job opportunities. Also such systems enable security of supply rapidly growing demand in the last decades, the infrastructure and reduce import dependence. Designing effective energy system relied on liquid fuels and different kinds of modes and needs the will consequently result in considerable less energy for covering transport sector represent a challenge for implementing renewable the same demand. energy sources. At the moment, oil and oil products cover more The increased need for power balancing introduced the power- than 96% of energy needs in transportation [1] and it is the only fuel to-gas technology. Power-to-gas refers to a system in which elec- that can meet the demand. The transport sector accounts for about tricity is converted into hydrogen by using water electrolysis. The 19% of global energy use and for 23% of energy-related carbon di- produced hydrogen can be stored, converted to methane and oxide emissions. Under current trends, transport energy use and reconverted into electricity if needed. An overview of power-to-gas CO2 emissions are projected to increase by nearly 50% by 2030 and power plants was given in Ref. [6]. Most of the power-to-gas pro- more than 80% by 2050 [2]. Reducing reliance on oil and oil prod- jects are in operation for a short while, with the exception of Ger- ucts in the transport sector is a daunting challenge [3]. Encouraging many that has put great emphasis in this technology. With two the strong decarbonisation of transport could lead to energy se- finished long run projects and five projects currently in the plan- curity which is an important goal for sustainability. While most ning stage, Germany could be considered as a leader in this concept. sectors have been taking measures to reduce CO2 emissions and However, it may not be the first option for integrating fluctuating shifting to renewable energy sources, the emission share for power from renewable energy sources in the electricity grid in the transportation has been steadily increasing. smart energy system [7]. The challenge of integrating transport sector in a 100% renew- able system goes along with integration of high shares of inter- mittent renewable sources and minimizing biomass consumption * Corresponding author. E-mail addresses: [email protected] (I. Ridjan), [email protected] through all energy sectors. This is demonstrated in a previous (B.V. Mathiesen), [email protected] (D. Connolly), [email protected] (N. Duic). studies relating to 100% renewable energy systems, in terms of 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.01.046 I. Ridjan et al. / Energy 57 (2013) 76e84 77 overall system [4] and heat sector [8]. Research show that is not just Recommendable scenario CEESA 2050, with the aim to minimise biofuels alone that cannot solve the transformation of the transport the biomass consumption in the transport sector to preserve it for sector to renewable energy, it cannot be dealt with using just one other sectors. specific measure or technology, but instead it has to be analysed in coherent transport scenarios [3]. 3. Solid oxide electrolyser cells (SOEC) While electricity is very important in the transformation of the transport sector, it cannot be used for all modes of transport and the There are several existing research and development projects on need for liquid fuels is inevitable. In this paper the conversion of SOECs in Europe. The main research centres for SOEC are located in electricity into some form of synthetic fuels is proposed. The term Denmark [11,12]. SOECs can operate as a fuel cell or as an electro- “synthetic fuel” relates to fuel made by using electrolysis as a base lyser. The difference between the two modes of operation is that in process and a source of carbon to produce liquid hydrocarbon. a fuel cell mode, the cell converts the chemical energy from a fuel Production cycle of synthetic fuels intertwines the heat and power into electricity through a chemical reaction while in electrolysis sectors, with the transport sector by using carbon capture and mode the cell produces fuels such as H2 and CO. The topic of in- recycling at a biomass power plant, thus providing carbon source terest for this analysis is electrolysis mode. The advantage of solid for electrolysis. The implementation of electrolysers in the trans- oxide electrolyte is that it conducts oxide ions, so it can oxidize CO port sector does not only provide synthetic fuels for transportation, and reduce CO2 in addition to H2/H2O. This cannot be done with it also provides an option for regulating the energy system. other types of cells, like proton exchange membrane (PEM) or þ Therefore, electrolysers possibly represent a good solution for alkaline cells, because their electrolytes conduct protons (H ) and À balancing and storage in renewable energy systems. hydroxide ions (OH ) respectively. SOECs operate at high a temperature (around 850 C) which has 2. Methodology both a thermodynamic advantage and an advantage in reaction rates. One of the benefits of high temperature electrolysis is that The aim of this study is to create alternatives for supplying the part of the energy required for splitting reactants is obtained in the transport sector with liquid fuels by measuring primary energy form of high temperature heat, enabling the electrolysis to occur supply, biomass consumption, system flexibility, and socio- with a lower electricity consumption. The electrolysis process is economic costs. The methodology for analysing synthetic fuel endothermic i.e. it consumes heat. High temperature electrolysis implementation can be divided into four steps: data collection, thus produces almost no waste heat, resulting in very high effi- technology and fuel review, energy system analysis, and a feasi- ciency, significantly higher than that of low-temperature electrol- bility study. ysis. Operating at high temperature results in faster reaction Very little literature has been identified relating to the imple- kinetics, which reduces the need for expensive catalyst materials mentation of SOECs in the future energy systems, given that the that is typical for low temperature electrolysis. While water elec- literature mostly focuses on materials, performance, and durability trolysis has been highly investigated thoroughly, electrolysis of CO2 of the electrolysis cells as well as the modelling of SOEC stacks. is reported on a smaller scale [13]. If steam and CO2 electrolyses are Different energy system scenarios, which include SOECs, are pro- combined in a process called co-electrolysis, the produced syn- posed in this study. This is followed by a review of the individual thetic gas, or shortly “syngas” contains varying amounts of carbon stages of the production cycle. Mass and energy balances are monoxide and hydrogen. The high operating temperature and high formed based on the chemical reactions of fuel production. A pressure, which provides further efficiency improvements, enables separate energy/mass flow diagram for each pathway outlining the the integration of a catalyst to convert the synthetic gas to synthetic electricity, biomass, CO2 and water needed for producing 100 PJ of fuel. The heat generated in the catalytic reaction can be therefore the primary fuel are then presented. utilized for steam generation and recycled in the system for elec- The overall energy system analysis and the feasibility studies trolysis [14].